I think i'll summarize what people have been saying in another post.
First and foremost before you start mapping you should realize that your first maps will probably not be publicly released as they will look and play horribly. Mapping will take a while to learn and you have to put in a lot of hours to get good at it.
With that said there are a lot of resources available for you to learn from. I found reading tutorials, especially at the beginning to be not as help as video tutorials. Youtube has a bunch, I really like Phillips Tutorials and VAAMaps. Check out Phillips first.
There is a stick at the top of the forums which has many resources available to you. After you watch some videos and read some tutorials I found two sites to be incredibly helpful. The first is the Valve Wiki the second is the Interlopers Website. I hope that helps.
I would like to mention that you should get your designs on paper first before you start mapping. Before producing my current map I think I spent 20-30 hours researching maps, architecture, drawing my designs before I even started to learn how to map.
Good luck with your map!